Re: [Vala] How to write float and double values into a binary file?

[Jonathan Ryan <jryan curious-os com>]

http://library.gnome.org/devel/glib/stable/glib-GVariant.html 
<http://library.gnome.org/devel/glib/stable/glib-GVariant.html#g-variant-builder-add-value>
http://www.valadoc.org/glib-2.0/GLib.Variant.html
Might be of some use.

Jonathan Ryan


On 12/17/2010 05:10 PM, Phil Housley wrote:
> On 14 December 2010 19:27, Sepehr Aryani<sepehr aryani gmail com>  wrote:
> > I'm wondering how to write a float or double value into a binary file.
> This is one of those things that is actually a lot harder than it
> sounds. One issue is that there isn't a universal standard for
> representing floats in binary, so instead you just have to pick a
> format, and remember to use the same one when you read. In a C app,
> that normally means just using the system's format, and hoping to read
> back on a similar system.
>
> In order to achieve that, you just cast the float to a void* (casting
> in C doesn't actually change any data, just affects compilation,) and
> write the correct number of bytes from that pointer. The write()
> method in FileStream is really just stdio's fwrite, and will let you
> do that. Except that unfortunately it doesn't really map well into
> Vala, which wants to view the void* as an array of bytes, and so the
> casting thing doesn't work out well.
>
> The best answer I can give is to find/make some sort of serialisation
> routine/library. Possibly there is something already built on GIO's
> richer stream classes. Either that or use text as a defacto
> serialisation format, and do the printf/scanf thing. Someone else
> might be able to suggest a more direct solution...
>
> Aside: The following doesn't work:
>
> public void main() {
>         float flt = 9.12f;
>         var output = FileStream.open("./data.txt","w");
>         output.printf("%f", flt);
>         float in_flt = 5f;
>         var input = FileStream.open("./data.txt", "r");
>         int read = input.scanf("%f", out in_flt);
>         message("%d %f", read, in_flt);
> }
>
> Because in the generated C, the output file isn't closed before
> attempting to read. This is a sort-of-bug, as there is no way to
> explicitly close a FileStream...
>
> > Well, 'FileStream.write' api looks like below:
> >
> >           public size_t<http://valadoc.org/glib-2.0/size_t.html>  *write* (
> > uint8<http://valadoc.org/glib-2.0/uint8.html>[] buf,
> > size_t<http://valadoc.org/glib-2.0/size_t.html>  size
> > = 1)
> >
> > I have to cast my floating point number into integer which in my case it is
> > useless...
> > In fact it seems impossible to me, because i failed to do that, Since write
> > function
> > needs an array of one-byte-values and I have a float value!!!
> >
> > If I use 'FileStream.printf' function, it writes data into file in ascii
> >   format.
> > this is my code sample:
> >
> > public void main(){
> > float flt = 9.12f;
> > var output = FileStream.open("./data.bin","w+b");
> > output.printf("%f", flt);
> >   float? in_flt = null;
> >         var input = FileStream.open("./data.bin", "r+b");
> > input.scanf("%f", in_flt); // !!!error
> > }
> >
> > I need a way write in floating point format and read in the same format,
> > without casting...
> > What should i do?
> >
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